This application takes priority from U.S. Provisional Patent Application Ser. No. 60/648,597, filed on Jan. 31, 2005.
1. Field of the Invention
The present invention relates generally to apparatus and methods for transmitting electromagnetic signals from a wellbore location to the earth's surface. In particular aspects, the invention relates to electromagnetic telemetry tools wherein an insulating connector or joint is used in the drillstring to form a dipole antenna for the telemetry tool.
2. Description of the Related Art
During drilling of wellbores in earth formations for the production of oil and gas (hydrocarbons) or other wellbore operations, such as completion etc., data or signals from a variety of downhole sensors carried by the drillstring or other metallic tubulars is processed downhole and transmitted to the surface for further processing and use. A variety of data transmission apparatus and methods are used to transmit or telemeter data to the surface. Electromagnetic (“EM”) telemetry systems are often used for transmitting relatively large amounts of data during drilling and other wellbore operations.
The data transmitted can include data from sensors such as pressure sensors, temperature sensors, sensors that provide data relating to the borehole conditions and drill string condition, and from a variety of other sensors that provide information about the earth formations. Such sensors herein are referred generally as the measurement-while-drilling or “MWD” sensors. The formation evaluation sensors may include resistivity, acoustic, nuclear, and magnetic resonance sensors. Data from sensors relating to drilling direction and borehole condition, also, is transmitted.
In an EM telemetry system carried by a drillstring, encoded signals are generated by an EM transmitter (source) disposed in a drilling assembly. The transmitted signals travel to the surface via the earth formation. These signals are received by a receiver at the surface and deciphered by a control circuit or processor.
For drilling operations, the EM telemetry apparatus is typically carried by or made a part of the drilling assembly (also referred to as the bottomhole assembly or “BHA”). The BHA is conveyed into the wellbore by a metallic tubular, such as a drill pipe that is made up of jointed pipe sections or a coiled tubing. The metallic tubular is used as the dipole antenna for the EM telemetry tool. This is done by dividing or separating the drill string into two conductive portions or sections by an insulating joint or connector (also referred herein as the “gap sub”). The insulating joint or connector is typically placed near the top of the BHA, the BHA and the drill pipe each forming an antenna for the dipole antenna of the EM tool. The signals or data to be transmitted to the surface are provided to the EM telemetry tool by controllers and processors carried by the BHA.
The use of gap subs, or insulated connectors for electrical isolation is known. U.S. Pat. No. 5,138,313 issued to Barrington, for example, discloses an electrically insulative gap sub assembly wherein the outer surface of a drill pipe joint is covered by several molded “gap blocks” of the insulative material. This technique can be expensive and complex in construction. In addition, it can also be prone to damage within the wellbore.
U.S. Pat. No. 4,348,672 issued to Givler describes an insulated drill collar gap sub assembly that is used with a particular toroidal-coupled telemetry system. An insulated gap is formed between a pair of annular sub members by forming a gap between them and filling the gap with a dielectric material. To interconnect the gap sub within the drill string, adjoining sub members are essentially keyed to one another using hexagonal keying. In an alternative version of the device, subs are connected using an axially extending member that resides within an axially extending recess. Pins are used to lock the two subs together, and a dielectric material is disposed in a gap between them. In each case, axial bearing assemblies are necessary to help transmit force through the gap sub. A significant disadvantage to this type of arrangement is the requirement for special tooling to form the various keys or extensions and recesses to mechanically lock the components together.
Additionally, U.S. Pat. No. 5,163,714 to describes an insulated connector that has limited mechanical strength. Another type of insulating connector is described in U.S. Pat. No. 6,404,350 to (the “'350 patent”), wherein an insulating coating or layer is applied to one of the two conical mating surfaces of the connector separating the upper and lower portions. In the system of the '350 patent, the insulating coating is stressed when the mating parts are joined, due to contact pressure and further due to relative movement of the two mating parts. Such insulating joints can be a source of failure downhole.
The present invention provides an EM telemetry system that has an insulating connector or gap sub that addresses some of the problems associated with the prior art gap subs.